Most current optical disk devices such as DVDs employ high-frequency modulation to restrict noise generated by the laser diode, which is used as the light source thereof. This strategy is well known by persons skilled in the art, and therefore, the following description contains only essential features and not details.
When high-frequency modulation is used, the laser performs pulse emission. In other words, the emission waveform repeatedly alternates between emission and turned off states, as shown in FIG. 2. In this case, the laser pulse interval (frequency) and the ratio of the emission period to this (duty) are the parameters that are adjusted so that the noise due to returned light when conducting high-frequency modulation is minimized. Since the emission waveform of the laser is a shape like that shown in FIG. 2, supposing the bandwidth limitation due to the photo-diode for readout and the current to voltage converting amplifier was nil, the readout signal waveform would be a shape like that shown in FIG. 3. Hereinafter, the signal consisting of a readout pulse train shall be referred to as the pulse readout signal. The dotted line in FIG. 3 is the readout signal waveform obtained in the case that the laser is continuously oscillated at the same output as the peak of the laser pulse during high-frequency modulation. In other words, the shape of the upper envelope of the pulse readout signal becomes a readout waveform based on continuous light. Accordingly, by passing the pulse readout signal through a low-pass filter having a cutoff frequency sufficiently lower than the frequency of the high-frequency modulating current (HF frequency), it is possible to obtain the desired readout waveform. In current optical disk devices, this is achieved by bandwidth limitation using a system consisting of a photo-detector and current to voltage converting amplifier, and an analog equalizer. To give an example of modulating HF frequency, in the case of the Blu-lay Disc (hereinafter referred to as BD) system, the standard is approximately 400 MHz. This is entirely determined according to the type of laser used and the optical path length of the readout optical system, so it is thought that there is not a big difference between devices. On the other hand, the upper limit of the frequency band containing the readout signal within the basic readout speed (1×) is 16.5 MHz, since the shortest mark or space length is 2 T (T: channel clock cycle).
FIG. 4A shows an example of the readout signal spectrum during 6× readout of a BD system with a disk capacity of 25 GB. Further, FIG. 4B shows an example of the HF laser pulse train spectrum. Since it is a periodic signal, it consists of a line spectrum. Further, the HF frequency is set at 396 MHz; equivalent to the channel clock. In other words, the HF frequency is 6 times the upper limit of the readout signal spectrum. In this example, the laser pulse duty is 25%. According to the duty, the intensity ratio of each order of the line spectrum changes. FIG. 4C shows the pulse signal readout spectrum. The pulse readout signal waveform is the product of the readout waveform shown by the dotted line in FIG. 3 and the periodic HF pulse train, so its spectrum becomes a convolution of the readout signal spectrum and the pulse light spectrum. Characteristics of the pulse readout signal spectrum are that the intensity of the higher order readout signal spectrum is nearly equivalent to the intensity of the baseband readout signal spectrum, and it has a line-shaped spectrum. Therefore, when attempting to recover the readout signal, which is the baseband signal, out from the pulse readout signal by using the bandwidth limitation of the system consisting of the photo-detector and current to voltage converting amplifier, and analog equalizer, it is necessary to adequately separate the upper limit frequency of the readout signal and the HF frequency.    [Patent document 1] JP-A No. 77640/1996    [Patent document 2] JP-A No. 221758/1996    [Patent document 3] JP-A No. 230814/2002    [Non-patent document 1] Lathi, B. P., Modern Digital and Analog Communication Systems, Volume 1, HBJ, 1985, p. 223.